Ultra compact scanning system for a wide range of speeds, angles and field depth

ABSTRACT

A non-contact light beam scan system small enough to fit into a hand holdable wand, pen or calculator size terminal incorporates a novel scan element which can scan at rates of ten to hundreds of scans per second in one or two dimensions. The device is immune to low frequency vibrations and can scan large angles of 60 degrees or more. Automatic trigger circuitry enables it to be used equally well in hand held or fixed mount applications. The depth of operating range is extended with a novel focal system which is integrated with the light source. The entire scanning system for generating a beam, focusing the beam electronically, scanning the beam, collecting light from a target and converting it into electrical signals, and automatically generating a trigger signal can work with industry standard low or high speed bar code decoders.

This is a continuation-in-part of: U.S. patent application Ser. No.08/006,754 filed Jan. 21, 1993 entitled "Programmable Gyrating Scanner",now U.S. Pat. No. 5,469,291, which is a continuation in part of U.S.patent application Ser. No. 07/612,983 filed Nov. 15, 1990, issued asU.S. Pat. No. 5,187,612; and of U.S. patent application Ser. No.07/776,663, filed Oct. 15, 1991 entitled "Electro-Optical ScanningSystem With Gyrating Scan Head", now U.S. Pat. No. 5,371,347; and ofU.S. patent application Ser. No. 07/745,905 filed Aug. 16, 1991 entitled"Scanning Device For Scanning A Target, Scanning Motor For The DeviceAnd A Method of Utilization Thereof", now U.S. Pat. No. 5,422,741; andalso of U.S. patent application Ser. No. 08/114,646 filed Oct. 28, 1993entitled "Light Beam Scanning Pen, Scan Module For The Device And MethodOf Utilization", now abandoned.

BACKGROUND OF THE INVENTION

1.) Field of the Invention

This invention generally relates to light beam scanning systems andmethods of scanning printed indicia such as bar code symbols and otheroptically discernible materials from which it is desired to extractinformation.

2.) Description of Prior Art

In a beam scanner a small spot of light is focused upon and sweptrapidly across a target from which information is to be read. A bar codereader is an important commercial application of a light beam scannerand is referred to herein as a typical example of a specific applicationfor the present invention. In a bar code reader the light is usuallyprovided by a laser diode emitting red light. After reflection from thetarget, light is collected onto a photo electric converter such as aphoto diode whereupon the collected light is converted into electronicsignals representing features of the target.

Millions of portable commercial laser bar code scanners and contact typebar code reading wands presently in service operate at scan speeds onthe order of 40 scans per second or less. The large number of installeddecoders used with these are only compatible with scanning rates in thisspeed range. Portable laser scanners like those described in U.S. Pat.No. 4,387,297 typically resemble a personal hair dryer in size andshape. Although these scanners automatically scan a light beam, theymust be carefully aimed at bar code targets to be read, and must bepositioned within the proper distance range from the target where thebeam is sufficiently focused to resolve features of the target. A manualtrigger is then pulled with the index finger of the user and a scanattempt is made. The scan attempt can last for up to a second and duringthis time small adjustments in aiming and range can be made before theread attempt is automatically terminated by the device. Inexperiencedusers may try to pull the trigger and sweep the scanner past a targethoping to catch it in order to avoid aiming. However, due to relativelyslow start up times and the slow scan rates of presently available handheld scanners, the scan line may be too narrow or may simply zigzagabove and below the target, missing it entirely. Even experienced usersdepend on visual feedback to see that the scan line is properlyintersecting a bar code target, but no visual feedback on spot focus isavailable to the user of present scanners. Therefore trial and error isthe only way an operator may successfully position the scanner away fromthe target. In bright lighting conditions the red scan line is difficultto see, and color blind users may not be able to see the red scan lineat all. If scan speeds were increased beyond 40 scans per second thescan line would become even more difficult to see. Also, for dense barcodes having narrow lines and spaces, the useful depth of field for mostportable laser scanners is only a few inches. (For 6 mil bars and spacesthis distance is typically about three inches.) Focus mechanisms similarto those used in automatic cameras might be designed for beam focusing,but these are quite complex and slow and thus have not been successfullycommercialized. Some attempts have been made to adapt the hand holdablegun shaped scanners to fixed mount point of sale applications bydesigning stands for them whereby they are aimed at a counter top areaand operated in a continuous scan mode. These attempts have not workedwell because even greater skill is needed for an operator to align andposition the goods to be read. This usually proves more difficult andcumbersome than manually positioning the scanner. These exampleshighlight the long standing need for a small scanner which requireslittle orientation with respect to the target being read and one whichis equally suited to both fixed or hand held operation in existingsystems. The contact reading wands, which do not scan a beam, must beheld in contact with the bar code and manually drawn across it by theuser; they are notoriously poor at reading non-flat surfaces, andrepeated attempts to read a bar code are often needed. The advantages ofa miniature high speed non-contact bar code scanner which may be heldwith the fingers like a pen were pointed out in U.S. patent application08/114,646 of which the present application is a continuation-in-part.In that application, miniature beam scan components, hereafter referredto as "scan elements", for a pen size beam scanner are disclosed.Several of the miniature scan elements disclosed therein are capable ofscan rates in the range of hundreds to thousands of scans per second.These require special high speed decoders and are not compatible withmany of the already installed low speed decoders. Also disclosed arebeam scanner embodiments which are low speed but which require delicatetorsion springs to achieve low scan speeds.

In designing scan elements of reduced size, it becomes increasinglydifficult to produce scan rates which are relatively slow, on the orderof 40 scans per second. This is because as the moving components of thescanners such as mirrors, magnets and coils are reduced in size, theirinertia is also reduced. Furthermore, a restoring force is usuallyrequired for the oscillating scan components and is often supplied bymetal springs, torsion elements or the like; these devices, when reducedin size and operated with extremely small inertial loads, cause theresonant frequencies of the scanner to be high, thus tending to producehigh scan rates.

Achieving adequate scan angles on the order of 40 degrees also becomes aproblem in the design of miniature scanners with mechanical springs ortorsion elements. This is so because as the length of a spring elementis shortened, so is its range of travel, whereas the elastic limits ofmany materials used for springs may be approached well before they flexenough to produce sufficient deflection to achieve an adequate scanangle. Operation of such springs close to their elastic limit will leadto fatigue and early failure.

If one attempts to use an extremely thin spring material with a very lowforce or torsion constant to fabricate a small low frequency scanner, ithas been found that such a scanner may be very fragile and susceptibleto picking up unwanted external vibrations especially in portable handheld applications.

The present invention incorporates many principles disclosed in theprior patent applications of which the instant application is acontinuation-in-part and discloses new embodiments which are miniaturein size yet capable of a wide range of scan rates from a very low scanrate of less than ten scans per second to moderately high rates of overone hundred scans per second. At the same time these devices are ruggedand can easily achieve scan angles on the order of sixty degrees ifnecessary. In addition, the depth of operating range is extended with anovel focal system which is integrated with the light source, andspecial positioning and automatic triggering techniques are disclosedwhich make the present invention more versatile and equally suited toboth fixed mount and hand held operation.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an extremely compactnon-contact scanning system with high immunity to external vibration,suitable for incorporation into wand or pen size information readoutdevices as well as pocket calculator size terminals having a keyboardand display which can operate at low scan rates on the order of 40 scansper second.

It is another object of the present invention to provide a scancomponent for scanning a beam below a resonant frequency at a moreconstant speed than typical resonant scanners have over a range of scanspeeds and scan angles.

It is yet another object of the present invention to provide a miniaturescan device which will start quickly and stabilize to a specific scanangle with low settling time.

It is still another object of the present invention to provide a selfadjusting focus means with the light source and the scan component toform an integrated modular scan system.

It is yet another object of the present invention to provide a miniaturescan system which can be adapted for use in a non-contact pen or wandshaped bar code scanner which is compatible with decode equipmentoperating in the low speed range on the order of 40 scans per secondalthough it may scan significantly faster than 40 scans per second.

It is a further object of the present invention to provide low power,low voltage drive circuitry for the scan system of the present inventionwhich is adapted for portable battery operated equipment.

Still another object of the present invention is to provide visualpositioning and range aids to a user. Yet another object of theinvention is to provide a scan system with automatic triggering,particularly which is equally suited for fixed mount or hand held use

It is yet another object of the present invention to provide a compactscan system which can provide two dimensional scan patterns such asomnidirectional and raster patterns.

These and other objects of the invention will become apparent as thevarious preferred embodiments are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a compact scan element and a light source according to thepresent invention.

FIG. 2 shows a cross section view of the scan element shown in FIG. 1.

FIG. 3 shows a scan element with a peripheral permanent magnet andferromagnetic ring for improved performance.

FIG. 4 shows a scan element similar to that of FIG. 3 with only aferromagnetic ring.

FIG. 5 shows a scan element with its mirror oriented in the direction ofthe Z-axis.

FIG. 6 shows an axial or in-line scan element with a peripheral magnet.

FIG. 7 shows an axial scan element with a ferromagnetic ring.

FIG. 8 shows a two dimensional axial scan element.

FIG. 9 shows a suspension for use with the scan element of FIG. 8.

FIG. 10 shows an axial scan element integrated with other components toform an ultra compact light beam scanning system.

FIG. 11 shows a compact electrically focusable device for use with thecompact scanning system.

FIG. 12 shows a flexible focus suspension for use with the device ofFIG. 11.

FIG. 13 shows a compact in-line scan system with a light collector.

FIG. 14 shows a complete pen or wand shaped beam scanning system.

FIG. 15 shows an axial scan element which produces no lateral beamdisplacement.

FIG. 16 shows another axial scan element according to the presentinvention.

FIG. 17 shows a schematic diagram of a circuit for driving a scanelement.

FIG. 18a shows a scan line produced in the X direction.

FIG. 18b shows a scan pattern produced in two dimensions where themovement in the Y direction is less than that in the X direction.

FIG. 18c shows a scan pattern similar to that of FIG. 18b where the Yscan component has finally increased enough to scan a bar code.

FIG. 18d shows a two dimensional scan pattern similar to that of FIG.18c but with an aspect ratio having greater Y motion.

FIG. 19 shows a preferred juxtaposition of two mirrors in an axial scanelement similar to that of FIG. 8.

FIG. 20 shows a block diagram of the automatic triggering and visualfeedback system.

DETAILED DESCRIPTION OF THE INVENTION

While the following description may refer to a bar code reader and theitems being read as bar code targets, it should be understood that theinvention is applicable to other information readout devices andequipment requiring beam scanning as disclosed herein. Accordingly, asused herein, the term target refers not only to a bar code but to anyitem having optically discernible features which require detection anddiscrimination by means of scanning.

In the following disclosure a device for scanning a beam is referred toas a "scan element" and a scan element in which a beam enters an inputend along a first axis and emerges from the scan element scanning aboutan axis which is substantially parallel to the first axis is referred toas an "axial scan element".

Shown in FIGS. 1 and 2 is a scan element which has no mechanical torsionspring and need not operate at a resonant frequency. Mount 51 issuspended on shaft 70 so it may freely rotate thereon with minimalfriction. The mount is best made from a lubricated plastic. Such plasticcompounds are available as injection moldable plastics with PTFE and drysilicone lubricants in them to produce extremely low friction and lowwear properties. Shaft 70 is preferably made from stainless steel wireand has its opposite ends attached to bobbin frame 45. Mirror 52 ismounted on one end of the mount so that a light beam 62 may reflect fromit.

The shaft imparts ruggedness to the mounting of mirror 52 and magnet 50,and restricts the motion of mount 51 to rotational motion on the shaftso that the device when operated as a scan element is not susceptible topicking up unwanted vibrations which can result in disrupted scanpatterns.

A means of preventing mount 51 from sliding laterally on the shaft isprovided, such as a shoulder or bushing on each side of the mount, likebushing 71 shown in FIG. 1. The bushing is recommended but is omittedfrom the remaining drawings for clarity of view. An alternate mountingmethod is to journal the ends of the shaft so the shaft rotates with themount fixed to it. This will also prevent unwanted lateral sliding ofthe mount.

A sectional view of FIG. 1 is shown in FIG. 2 for clarity. Magnet 50,magnetized in direction M, is attached to the bottom of mount 51 in thecentral opening of coil 44. On the outer periphery of coil 44 is asecond magnet 53 which is magnetized in the same direction as magnet 50.The lines of force between the two magnets will thus cause magnet 50 tobe attracted to magnet 53 thereby automatically bringing it to alignmentin the equilibrium position depicted in FIG. 2. It will be recognized byone skilled in basic principles of magnetism that the position ofequilibrium for magnet 50 is that of its lowest potential energy stateand it will tend to orient itself into the position shown in FIG. 1 withthe coil axis 63 also oriented as shown, thereby providing a restoringforce for magnet 50 without the use of a mechanical spring. The presenceof magnet 53 also causes magnetic flux in the direction of arrow 56 frommagnet 50 to concentrate, and directs this flux so that a portion of itwill penetrate the windings of coil 44 rather than spreading outdiffusely or returning directly to the opposite pole of magnet 50.

Energizing coil 44 with an alternating current will cause magnet 50,mount 51 and mirror 52 to oscillate rotationally about shaft 70 with arocking motion thereby causing light beam 62 reflected from mirror 52 tobe scanned repetitively through angle X to effect a straight line scanabout equilibrium position 63 also represented by axis Z. It should bepointed out here that in the assembly of the scan element of FIG. 2 theposition of magnet 53 may be adjusted so that the neutral Z-axis isaligned as desired before magnet 50 is permanently secured in place.Furthermore by sliding magnet 53 in a direction out of the page, theforce between magnets 50 and 53 will be reduced thereby changing theresonant frequency of the scan element.

Of course it is possible to add another magnet across the diameter ofcoil 44 and opposite magnet 53 to gain even greater efficiency ofoperation if the power requirements for a miniature device constructedaccording to FIG. 2 justifies the extra part. Also it should be notedhere that coil 44 need not be circular; it may be square or oval or thelike but because of its simplicity a circular coil is illustrated inFIG. 1.

As an example, a scan element like that of FIG. 2 has been fabricatedwith a coil diameter of about 3/8 inch with a rare earth magnet 53having a diameter of 1/8 inch and thickness 1/64 inch, whereas a rareearth magnet 50 had a length and width of 1/8 inch and thickness of 1/32inch. In embodiments of these proportions it was found that the devicecould operate over a wide range of frequencies with a first oscillatoryresonant frequency occurring at about 160 scans per second. The devicecould also be operated in the broad band range down to less than 5 scansper second. If operated in the range below resonance it will scan at arelatively linear rate, following the applied wave form faithfully withthe angle of scan generally proportional to the current applied to thecoil requiring only about 20 milliwatts of power to provide a 40 degreescan angle at 40 scans per second. If the scan element is operated atresonance, it executes scans with a generally sinusoidal speed profileand reduced current. If the device of this example were fabricated witha stronger peripheral magnet 53, then it would be suited for operationat higher frequencies.

In the broad band mode of operation (at frequencies below resonance ),if a triangle wave form is applied to coil 44 of the scan element, thenthe resulting scanned spot will have a speed which is more constant thanthat of resonant scanners over most of the scan (except where it muststop and reverse at the end points of each scan). On the other hand, ifthe wave form has a frequency close to resonance, then the resultantspot speed will not be relatively constant but will reach a peak speedin the middle of the scan. The value of this peak speed ultimatelylimits the rate at which decoders can process data from a scanner. (Thepeak speed for a resonant scanner can be as great as four times that ofits lowest usable speed during a single scan.)

Thus a scanner which scans with a more constant spot velocity can remaincompatible with decoders while operating at higher scan speeds than dothose scanners which have non-constant scan rates and high peak spotspeeds.

Because the scan elements of the present invention can have relativelyconstant spot speeds over a substantial portion of their scan angle andspeed range, as explained above, they may be operated at substantiallyhigher speeds than 40 scans per second and still remain compatible withstandard industry decoders rated for average speeds of only 40 scans persecond.

It has also been discovered that when the scan element of FIGS. 1 and 2is operated below resonance at about 40 scans per second, it can startvirtually instantly, presenting a full width line scan with no humanlyperceptible starting time, overshoot or settling time. This is avaluable attribute in portable hand held equipment because at speeds of40 scans per second, slow starting, overshoot and undershoot are readilydetected by a human operator and are perceived as sluggishness. Severalknown scanners which are operated at a resonance of 18 Hz and produce 36scans per second have slow start characteristics and a user can see thescan line grow to its final width.

Turning now to FIG. 3, a further enhancement of the device of FIGS. 1and 2 is shown, wherein a ferromagnetic ring 54 has been added aroundcoil 44. The ring serves to provide a flux return path for the fields ofmagnets 50 and 53, thereby improving the energy efficiency of thescanner and providing shielding from external magnetic fields whichcould disturb the scanning characteristics of the scan element.

FIG. 4 shows another embodiment of the device of FIG. 3 but without theperipheral magnet 53, although the ferromagnetic ring 54 is retained inthe design. In this arrangement the ring 54 serves to direct flux in atleast one of the directions indicated by arrows 56 and 57 from magnet 50through opposite sides of coil 44, and also serves as a shield againstoutside magnetic fields. As was explained for the scan element of FIGS.1 and 2, it will also be recognized that the position of equilibrium formagnet 50 is that of its lowest potential energy state, and magnet 50will tend to automatically align itself into the position shown infigure 4 with the Z-axis also oriented as shown due to the presence ofthe ferromagnetic ring around coil 44. This position of minimum energyalso establishes the lowest reluctance path for the field of magnet 53.In addition, the ring contains the magnetic flux generated by the coilitself and serves to aid in guiding and concentrating this flux upthrough the center of coil 44 so that it can more effectively interactwith the magnetic field from magnet 50.(The ring need not completelysurround the entirety of the coil to serve its purpose. For example itmay be split so it can tightly hug the coil or it may have some materialremoved from a portion of it to provide some adjustability with respectto its flux directing properties by sliding or rotating it with respectto the coil and magnets.) When an alternating current is passed throughcoil 44 of the scan element of FIG. 4, it will produce a line scan as dothe scan elements previously described. It should further be pointed outthat it would be possible to substitute an electromagnet for magnet 53.

The scan element of FIG. 4 has fewer parts than do some embodimentsdescribed herein, and has several desirable characteristics as follows:

(a) it does not have the extra peripheral magnet as do the scan elementsof the previously described embodiments and is therefore less expensiveand can be somewhat smaller,

(b) it will have a very low resonance frequency since the stronger forceof a peripheral magnet is not present,

(c) although not quite as efficient as some of the previously describedembodiments, it can be operated with little power at or near resonanceyet it will still start quickly with very short settling time.

Now turning to FIG. 6 it is seen how the scan elements of FIGS. 1, 2, 3,4 or 5 may be adapted for operation as an inline scan element, hereafterreferred to as an "axial scan element". In FIG. 6 an embodiment of thepresent invention is shown wherein an input beam 62 enters the device atone end and emerges from the device scanning about axis 63 which isgenerally parallel to beam path 62, i.e. essentially in the samedirection as the incident light beam that entered it. With thisembodiment it is possible to create an extremely compact and simplein-line optical layout for a scanner making possible a non-contactscanner shaped like a pen or wand. Such pen and wand scan systems havebeen described in patent applications of which the present invention isa continuation-in-part. However, for the pen or wand configurations theaxial scan elements disclosed herein are preferred for, but not limitedto, scan rates on the order of 40 scans per second. These versatile scanelements are rugged, and small and may be operated at low speeds whichare compatible with the many low speed decoders already in service orthey may be operated at speeds several times higher.

For a scanner employing the axial scan elements of FIGS. 6 and 7, alight beam from light source 60 is shaped into a narrow beam 62 bypositive lens 61 and directed through an opening 82 in the scan elementwhereupon the beam is reflected by first mirror 80 after which it isagain reflected by second movable mirror 52 and finally emerges from thescan element traveling in the same general direction which it originallyhad with only a slight lateral displacement. When coil 44 is energizedwith alternating current, beam 62 is scanned repeatedly through angle Xabout axis 63 by employing the same electromagnetic principles as in thepreviously described embodiments. The axial scan embodiment isconsidered the most compact embodiment and therefore the best embodimentfor pen shaped non-contact scanners. It should be noted that in accordwith elementary principles of optics, the axial scan element may beoperated in reverse by first reflecting an input beam from the movablemirror 52 then from the fixed mirror 80.

Now turning to FIG. 16 another variation of an axial scan element isseen in which coil 44 is displaced along its axis 63 with respect tomovable magnet 50. Although this embodiment may not be quite asmagnetically efficient as some of the embodiments previously discussed,it nevertheless it has been found to work admirably well, requiring lessthan 10 milliamps at less than 4 volts to scan a beam over 40 degrees at40 scans per second. A version of it has been built and operated withoutring 54. An advantage of this latter configuration is that it may bemade even smaller in diameter than the element of FIG. 6, for example,while still retaining the quick start with minimal overshoot capabilityat low speeds.

It is possible to eliminate the slight lateral displacement of the beammentioned in the description of FIG. 6. Referring now to FIG. 15 anembodiment is shown in which the beam is not laterally displaced. Beam62 is directed at mirror 86 which is 50% reflective and therefore 50%transmissive. The portion of the beam passing through mirror 86 thenpasses through hole 91 provided in magnet 93 and encounters movablemirror 87 which is also 50% transmissive whereupon 25% of the originalbeam passes directly through it as a fixed non-moving beam which issuitable for an aiming aid. Another 25% of the beam is reflected back tomirror 86 but this portion of the beam is being scanned due to the factthat it has been reflected from moving mirror 86. Fifty percent of thisportion of the beam will pass through mirror 86 and will be lost and 50%of it, or 12 1/2 % of the original beam will be reflected again tomirror 87 whereupon half of this or 6 1/4 % of the original beam finallyemerges as scanned beam 63. A visible laser diode with an output powerof 5 milliwatts may be used for the light source 60, thereby deliveringa scanned beam output of 0.3125 milliwatts, which is adequate for anoncontact bar code scanner capable of reading targets at a distance ofone to two feet. Since the application of power from the laser isintermittent, requiring only a fraction of a second to scan a target,power consumption is not a critical issue even for portable batterypowered equipment. Shafts 70A and 70B, coil 44 and peripheral magnet 53function in the same manner as for other scan elements of the presentinvention which have been previously described.

MULTI LINE SCANNING

It is desirable in some target scanning applications to generatemultiple scan lines in order to cover a two dimensional area. Someexamples are: (a) where it is desirable reduce the requirement forcritical alignment of the target to the scanner at check out countersfor example, (b) where damage or dirt is present on the target and multidimensional scanning is useful for making different passes across thetarget, (c). where sophisticated two dimensional scanning needed to readtwo dimensional bar codes especially in portable applications. These andother requirements are fulfilled by the present invention as shall nowbe explained.

FIG. 8 shows an axial scan element capable of scanning a beam over twodimensions. In the device of FIG. 8 portion 95 includes an axial scanelement as has already been described. Beam 62 reflects from opticalelement 79 which is disposed to reflect the beam again from movablemirror 52 after which it exits the device moving about neutral axis 63.A mirror is considered the simplest and best choice for optical element79 but it could also be a refractive element or the like. Opticalelement 79 is caused to move with a rocking motion on an axissubstantially orthogonal to the rocking motion of mirror 52 which rocksabout shaft 70. Optical element 79 is mounted to flexible suspensionelement 84 of FIG. 9. Flexible suspension element 84 is actually capableof both X and Y compound movement and was described in U.S. patentapplication Ser. No. 08/114,646 of which the instant application is alsoa continuation-in-part. For the utmost control of a two dimensionalraster pattern the juxtaposition of mirrors 52 and 79 as depicted inFIG. 19 is desirable wherein the planes of the mirrors are arranged at45 degrees with respect to one another. Thus each mirror is onlyrequired to execute single dimensional motion and a raster scan 105 canbe produced with excellent control.

Turning back to FIG. 8 now, area 83 of suspension element 84 allowsunimpeded passage of beam 63. Magnetic element 67, also attached tosuspension 84, can be a permanent magnet which preferably is magnetizedin a direction mutually perpendicular to axis 63 and the direction ofmagnetization of magnet 50. Magnetic element 67 moves reflective element79 and scans beam 62 in response to current in coil 74. Thus when bothreflective elements 52 and 79 are moved, a two dimensional scan isobtained which can scan an area.

It is important to note here that the device of FIG. 8 can be operatedas a two dimensional scanner without coil 74 and thus constitutes animportant embodiment of the present invention. If flexible suspensionelement 84 is made from relatively stiff, flat beryllium copper alloy,for example, it can have very high resonant frequencies of hundreds ofHertz in more than one direction with little response at lowfrequencies. By superimposing a high resonant frequency on a lowfrequency and introducing both frequencies into coil 44 alone bothreflective elements 52, and 79 will oscillate orthogonal to andessentially independently of one another, thereby producing a twodimensional scan pattern. A raster pattern is an extremely useful twodimensional pattern which the device of FIG. 8 can produce and is wellsuited for reading two dimensional bar code. Ideally such a raster isgenerated when the wave form delivered to coil 44 consists of a lowfrequency triangle wave to produce low speed linear scans by mirror 52in a first direction and very high speed scans in a second directionperpendicular to the first. This results in many closely and evenlyspaced scan lines forming the raster. FIG. 19 shows the juxtaposition ofmirrors 52 and 79 which are positioned at about 45 degrees with respectto one another for efficient raster production.

MULTI LINE OMNI SCAN METHOD

A novel method for scanning a randomly oriented bar code target withoutthe need to carefully aim the scanner can also be achieved with the scanelement of FIG. 8. By amplitude modulating the angular excursions of atleast one of the mirrors 52 or 79, of the 2-D scan element in FIGS. 8 or19 a continuously variable aspect ratio scan pattern is produced asshown in the sequence of FIGS. 18a, 18b, 18c, and 18d. In this sequence,bar code target 202 is shown with no special orientation with respect tothe X-Y plane but which will be scanned successfully using the followingmethod without the need to reorient it.

The first FIG. 18a in the sequence shows a scan line in the X directiononly, and of course code 202 is not successfully scanned. A briefinstant later a Y component is added to the scan pattern by applying anappropriate drive signal to the two dimensional scan element of FIG. 8as previously described. Still the new scan pattern is not yetsufficiently expanded to successfully scan target code 202. Finally theY amplitude is increased even more, as shown in FIG. 18c, and code 202is successfully scanned by scan line 206a which has changed its angularorientation as the Y scan component was increased. Then the Y scanamplitude is increased even more as seen in FIG. 18d to cover apredetermined area, whereupon the sequence is reversed. This process ofscanning is repeated in a continuous manner many times a second, and asa result, targets may be placed randomly oriented in the target area andsuccessfully read. This technique of scanning is extremely useful forpoint-of-sale or portable scanners to increase productivity and toreduce frustration of the user. Of course the X- direction scanamplitude may also be modulated similarly to achieve usefulomni-scanning according to this method.

Techniques for amplitude modulation of wave forms are well known andneed not be of high precision or fidelity to be effectively employed inthis novel method of scanning.

Also it should be pointed out that if, in the scan element of 5 FIG. 8,mirror 52 were held fixed and mirror 79 were caused to resonate withcomponents of motion in the X and Y directions at appropriatelydifferent frequencies, these frequencies could also be amplitudemodulated to produce expanding and collapsing lissajous figures toachieve time varying omni scan patterns according to the method justdescribed.

DRIVE CIRCUITS FOR USE WITH THE SCAN ELEMENTS

FIG. 17 shows a schematic diagram of a circuit for powering the scanelements of the present invention. Since the best mode of operation isconsidered to be at a frequency below resonance so that the scan elementmay sweep the beam at a generally constant speed, a triangle wave formis preferred. Also, for portable equipment, the circuit is designed todrive the scan element at low voltage from a single ended battery powersupply.

In the circuit of FIG. 17 amplifiers U1A and U1B are currentdifferencing amplifiers. A square wave is produced at the output of U1Aat a frequency equal to the number of scans per second desired. Thesquare wave is integrated by amplifier U1B to form triangle waves of thesame frequency. The current differencing amplifiers allow integration ofthe square wave without the need for a double ended power supply. Thesquare waves are fed into a trigger flip-flop to yield square waves at Qand Q' which are 180 degrees out of phase with one another and have onehalf the frequency of the original square wave from U1A. The outputs Qand Q' drive the two legs of an H-Bridge consisting of low on resistanceMOS FET drive transistors Q10, Q11, Q12, and Q13. The triangle wave fromU1B is one diode drop above ground so it is fed into the base oftransistor Q1 which acts as an emitter follower to buffer the signalfrom U1A. The base-emitter voltage drop of Q1 causes the bottom peak ofthe triangle wave appearing at the emitter resistor R50 of Q1 to bereferenced to ground potential. The emitter resistor of Q1 is apotentiometer which feeds a triangle wave of the desired amplitude tothe input of positive buffer amplifier U2 which supplies drive signal tothe drains of Q10 and Q11. As the two branches of the H-Bridge arealternately turned on and off at the gates of the drive transistors analternating current triangle wave form appears at the output of theH-Bridge to drive the scan element at a frequency equal to one half thesquare wave frequency of U1A. Also since industry standard decodersusually require a start- of-scan signal to signify the start ofscanning, either Q or Q' or both of these signals will suit thispurpose. This circuit therefore supplies the necessary alternating drivecurrent to operate the scan element efficiently from a single endedpower supply and also delivers the necessary start-of-scan signals forstandard decoders.

BEAM CONDITIONING

Extended depth of operating range is provided by a compact continuouslyvariable focal element or focuser which is now disclosed.

In FIG. 11 a light source 60 is preferably in a ferromagnetic housing asare the packages of some of the newer visible laser diodes such asToshiba part number TOLD 9221M. A focus coil 450 surrounds theferromagnetic housing of the light source 60 so that the housing formsthe core of an electromagnet. A resilient flexible suspension 455 madefrom flat ferromagnetic magnetic material such as steel about 2 milsthick suspends miniature beam converging optical element 454 above thehousing of light source 60. The simplest form of element 454 is apositive lens but a hologram, axicon, or other non-imaging beamconverging element could be adapted. For simplicity, element 454 will bereferred to as a lens.(Suspension 455 need not be completely magnetic;just having its central portion magnetic would also work. Also a thinmagnet with a hole in its center for the beam to pass through may alsobe added to the center of suspension 455 to increase electricalefficiency.) Typically lens 454 is chosen to have a short focal length,on the order of about 3 mm. Upon energizing coil 450, suspension 455 ispulled in toward the housing of the light source 60 which moves lens 454toward the light source resulting in focusing the beam at differentpoints such as f1, f2, f3 down range along axis 470 depending upon themagnitude of the current introduced into coil 450. A particularadvantage of this focus element is that it does not waste light as wouldoccur if a very small stop aperture were used to gain field depth.

It has been found that a structure for resilient optical suspension 455such as the one illustrated in FIG. 12 is ideal for suspending positivelens 454 because it preferentially moves in the direction of axis 470while minimizing tilting of the lens. In addition to holding lens 454suspension 455 can be fabricated with an aperture 459 to gain additionalcontrol over the spot size and shape as desired. The axial symmetry ofthe focus device of FIG. 11 makes it ideal for incorporating into theinventive in-line modular scan system shown in FIG. 10. Focus coil 450with suspension 455 attached is typically fitted around magnetic lightsource housing 60a so that the beam is focused at a first position alongaxis 470 and is then cemented in place.

Application of an asynchronous drive current to coil 450 which variesslowly with respect to the scan rate of the scan element results inmoving the focal point of the beam down range along axis 470 while thebeam rapidly scans. The net result of this scan method is that a targetsomewhere down range will eventually come into adequate focus during ascan sequence. For example, if the scan element rate is 40 scans persecond and the focal element is driven about 4 Hz, or back and forth 8times per second, then a target at a given location will be insufficient focus about 8 times a second. Since only one good scan isneeded and one second is not a long scan period, the perception of auser will be that the scanner has a great depth of scan range. Atriangle wave form generator such as the one of FIG. 17 would also bewell suited for use with the focus coil.

AUTOMATIC TRIGGERING AND RANGE INDICATION

In order for a decoder to receive information from a bar code scanner, atrigger signal is supplied to it which is simply a logical 1 or 0 duringdata transmission. The trigger can be a manual switch as is found oncontact wands or portable type scanners. However manual triggers are notwell suited for fixed mount operation.

In one embodiment of the present invention an automatic internalelectronic trigger is employed which allows the present invention to beused for either fixed mount or hand held operation as desired. Inaddition the trigger system now described gives the user both targetpositioning and range feedback even in strong ambient lightingconditions. A power switch S1 operatively associated with the scanner asshown in FIG. 20, turns the scan system 300 ON when activated but doesnot energize beam scan element 308. Typically switch S1 is located onthe housing of the scanner and is operable both as a momentary orconstant on switch. It may be mechanical as found on some flashlights ora touch type switch such as a capacitance switch so that by touching thehousing of the scanner it will go on.

When switch S1 is activated light source 60 is caused to blink rapidlylike a strobe by blink control 312, preferably with a low duty cycle andat a rate humanly perceptible such as 30 blinks per second. It is wellknown that a blinking signal is easier to spot than a constantly onlight signal. When the blinking commences the beam is not yet scanned.Thus an intense spot of light is created which can be seen even instrong ambient light like that of a laser pointer at close range so thatthe mutual position of target and beam can be brought into position forscanning. This feature allows a target to be positioned in front of thescanner or the scanner to be aimed at the target with equal convenienceand at the same time save power. The blink mode is typically timed toprovide at least a few blinks before scanning is commenced. During theblink mode, the photo amplifier 304 and digitizer circuit 306 is alsoturned on by the power switch S1 and detects reflected blinking lightfrom the target. When the detected light is strong enough as detected byrange sensor 314 for the target to be in range, an internal electronictrigger switch 318 such as a transistor switch is activated to alert thedecoder that information is on the way. At the same time the scanelement is turned on and scans the beam, and the light source ceasesblinking and is put into a constant 0N mode by blink control 312. If thetarget is too far away and out of range, the blinking simply continuesas long as power switch S1 is on, indicating to the user that the targetis out of range. It should be pointed out that triggering is not onlyautomatic but depends on the scanner and target to be in the correctrange for successful scanning. Since human operators can perceiveintervals on the order of about 50 milliseconds initiation of adequatescan angle longer than about 100 milliseconds is perceived as sluggishor "non aggressive" thus techniques like the one just described wouldhighlight the slow start up times of prior art scanners. Because thescan elements of the present invention start virtually instantly, andvisual range feedback to the user is provided by the present invention,it is practical to implement the above described system without theperception of having to wait for the scan line to reach its normal scanangle. Furthermore, the time consuming trial and error methodsassociated with finding the correct range for the prior art scannersmake the scanner of the present invention more productive and admirablyuseful for either hand held or fixed mount operation.

Since the methods of transistor switching and timing circuits areelementary and well known there is no need to describe the many ways ofimplementing the above circuit function.

INTEGRATED SCAN SYSTEM

Now turning to FIG. 10 a scan element 99 is shown installed into amodule 100 comprised of light source 60, focus element 61, which in thiscase is a positive lens for converging the beam from the light source,and a drive circuit 35 for powering the module. The focus element 61 maybe replaced with virtually no space penalty by the novel electricallymovable focus element shown in FIG. 11 for greatly improved depth offield operation.

Turning now to FIG. 13 a scan module is shown wherein light is collectedand concentrated by coaxial optical element 16. This type of lightcollector was described in U.S. patent application Ser. No. 07/776,663of which the present application is a continuation-in-part. Lightcollector 16 is preferably made from transparent plastic with a highindex of refraction and acts to funnel light onto photoelectricconverter 14 by total internal reflection rather than by imaging. Anoptical filter 16B filters out unwanted wavelengths of light. Thusbecause of its hollow cylindrical shape light collector 16 can fitaround module 18 which contains components similar to those shown inFIG. 10. Module 18 has been fabricated with a diameter of 3/8 inch andlength 3/4 inch and the overall diameter of light concentrator 16 can beabout 1/2 inch so that the module of FIG. 13 can fit into a pen or wandas shown in FIG. 14.

An entire scanning system for generating a beam, focusing the beamelectronically, scanning the beam, collecting light from a target andconverting it into electrical signals can compactly be housed into thenon-contact beam scanning pen or wand as shown in FIG. 14. The system isalso so small that it may be incorporated into a compact pocket sizecalculator type terminal with a key pad and display. Such a scanterminal when integrated with non-contact thermometers presentlyavailable and finger type blood pressure sensors would make a terminalideally suited for the health care industry.

In addition to the advantages of the system already described, the quickstart features and range feedback features of the present inventionprovides even further advantages. For example in prior art terminalssuch as described in U.S. Pat. No. 4,758,717 and U.S. Pat. No. 5,130,520a micro processor is used to determine a successful scan and toautomatically terminate scanning when complete. Such methods are usefulin slow speed prior art scanners which suffer from slow startup andlimited range of operation. The user must try various ranges and adjustaim until the terminal tells him, usually with a beep, that scanning wasfinally successful then turns itself off to conserve power. With thehigh speed, quick start, auto ranging scanner embodiments of the presentinvention it is no longer necessary to employ a micro processor toterminate scanning. Since the probability of instantly reading anddecoding a target is much higher with the present invention, the scannercan be activated in the portable terminal mode for about 250milliseconds then terminated with a simple RC time out circuit duringwhich time many successful scans will have occurred.

While various embodiments of the present invention have been set forth,variations will no doubt occur to those skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A light beam scanning system for scanning atarget to read information therefrom, said system comprising a scanelement for receiving a light beam and directing it toward a target as ascanning beam with an angular oscillatory movement of said beam about aneutral axis, said scan element comprising:a coil for carrying electriccurrent, said coil having a coil axis and an opening along said coilaxis; a shaft having a shaft axis disposed substantially perpendicularto said coil axis; a magnetic element mounted for rotation about saidshaft axis in response to current in said coil and producing a magneticfield oriented generally perpendicular to said shaft axis; a firstbeam-directing optical element which is coupled to said magnetic elementfor rotation therewith about said shaft axis; and a ferromagneticcircuit including at least one ferromagnetic element disposed withrespect to said magnetic element so as to establish a minimum energyrotational orientation of said magnetic element in which said magneticfield of said magnetic element is oriented generally perpendicular tosaid coil axis.
 2. The system of claim 1, wherein said ferromagneticcircuit includes a ferromagnetic element which at least partiallysurrounds said coil.
 3. The system of claim 1, wherein saidferromagnetic circuit includes a magnet.
 4. The system of claim 3,wherein said ferromagnetic circuit includes a ferromagnetic ring whichsubstantially surrounds said coil.
 5. The system of claim 3, wherein themagnetic field of said magnet is oriented generally parallel to themagnetic field of said magnetic element.
 6. The system of claim 1,wherein said scan element directs said light beam about a neutral axiswhich is generally parallel to the light beam received by the scanelement.
 7. The system of claim 1, wherein said first beam-directingoptical element is a first mirror, further including a second mirror,said first and second mirrors being disposed so that an incident lightbeam reflected from one of said mirrors toward the other of said mirrorsis reflected from said other of said mirrors in a direction which isgenerally parallel to said incident beam.
 8. The system of claim 1,further including a single ended power supply and a drive circuitoperable from said single ended power supply to produce a waveform fordriving said coil.
 9. The system of claim 1, wherein said first opticalelement is a mirror which in said minimum energy orientation is orienteddiagonally with respect to said coil axis.
 10. The system of claim 1,wherein said ferromagnetic element includes a portion adapted to beadjusted in position so as to establish the direction of the neutralaxis about which the scanning beam is directed.
 11. The system of claim1, wherein said scan element directs said light beam about a neutralaxis which is generally parallel to said coil axis.
 12. The system ofclaim 1, wherein said coil is wound on a bobbin, and said shaft ismounted to said bobbin.
 13. A scan element for receiving an input lightbeam along an input axis and directing it as a scanning beam with anangular oscillatory movement of said scanning beam about a neutral axis,said scan element comprising:a coil for carrying electric current, saidcoil having a shape which is generally tubular with a coil axis, anopening along said coil axis, and an outer peripheral region; a rotormounted for rotation about a rotation axis, said rotation axis beingoriented generally perpendicular to said coil axis, said rotor includinga rotor magnet producing a magnetic field oriented generallyperpendicular to said rotation axis, said rotor magnet imparting torqueto said rotor about said rotation axis in response to current in saidcoil; biasing means magnetically interactive with said rotor magnet forestablishing an equilibrium rotational orientation of said rotor andimposing upon said rotor, when said rotor is rotated from saidequilibrium orientation, a torque urging said rotor toward saidequilibrium orientation; and an optical assembly disposed to receive aninput light beam along an input axis and direct it as an output beam,said optical assembly including a first beam-directing optical elementwhich is mounted to said rotor so as to establish a neutral axis of saidoutput beam when said rotor is in said equilibrium orientation and torotate therewith about said rotation axis in response to current in saidcoil and thereby scan said output beam.
 14. A scan element according toclaim 13, wherein said neutral axis is generally parallel to said coilaxis.
 15. A scan element according to claim 13, wherein said input axisis generally parallel to said coil axis.
 16. A scan element according toclaim 13, wherein said neutral axis is generally parallel to said inputaxis.
 17. A scan element according to claim 13, wherein said coil axis,input axis, and neutral axis are generally parallel to each other.
 18. Ascan element according to claim 13, wherein the optical path of a lightbeam received and directed by said optical assembly includes a portionlying within said coil opening.
 19. A scan element according to claim18, wherein said optical path passes through said coil opening.
 20. Ascan element according to claim 13, wherein said biasing means includesa ferromagnetic element disposed in said outer peripheral region.
 21. Ascan element according to claim 13, wherein said biasing means includesa ferromagnetic element disposed so as to concentrate and direct fluxfrom said rotor magnet through said coil.
 22. A scan element accordingto claim 13, wherein said biasing means includes a biasing magnetdisposed in said outer peripheral region.
 23. A scan element accordingto claim 22, wherein said biasing magnet is disposed with its magneticfield oriented generally parallel to the magnetic field of said rotormagnet.
 24. A scan element according to claim 13, wherein said biasingmeans includes a ferromagnetic ring disposed in said outer peripheralregion.
 25. A scan element according to claim 13, wherein said biasingmeans includes a ferromagnetic element which is adjustably positionablein said outer peripheral region so as to adjust the magnetic interactionbetween said biasing means and said rotor.
 26. A scan element accordingto claim 25, wherein said adjustably positionable ferromagnetic elementadjusts said equilibrium orientation of said rotor.
 27. A scan elementaccording to claim 25, wherein said adjustably positionableferromagnetic element includes a magnet.
 28. A scan element according toclaim 25, wherein said adjustably positionable ferromagnetic elementincludes a ring of ferromagnetic material at least-partially surroundingsaid coil.
 29. A scan element according to claim 13, wherein said firstbeam-directing optical element and said rotor magnet are located so thattheir moments of inertia about said rotation axis at least partiallybalance.
 30. A scan element according to claim 13, wherein said rotationaxis is defined by a shaft.
 31. A scan element according to claim 30,wherein said coil is wound on a bobbin, and said shaft is mounted tosaid bobbin.
 32. A scan element according to claim 13, wherein saidrotor magnet is located along said coil axis.
 33. A scan elementaccording to claim 13, wherein said rotor magnet is located in said coilopening.
 34. A scan element according to claim 13, wherein said firstbeam-directing optical element is a partially reflective and partiallytransmissive element.
 35. A scan element according to claim 13, whereinsaid first beam-directing optical element is a mirror.
 36. A scanelement according to claim 35, wherein said mirror is orienteddiagonally with respect to said coil axis.
 37. A scan element accordingto claim 13, wherein said optical assembly includes a secondbeam-directing optical element, said first and said secondbeam-directing optical elements each including a reflecting surface,said reflecting surfaces of said first and said second beam-directingoptical elements being oriented generally parallel to each other.
 38. Ascan element according to claim 13, including a generally tubularmagnetic shield located in said outer peripheral region.
 39. A scanelement according to claim 38, wherein said generally tubular magneticshield is generally coaxial with said coil and at least aboutcoextensive with said coil along said coil axis.
 40. A method ofscanning a light beam to provide a scanning beam with an angularoscillatory movement of said scanning beam about a neutral axis,comprising the steps of:mounting a rotor, comprising a rotor magnet anda first beam-directing optical element, for rotation about a rotationaxis oriented generally perpendicular to the magnetic field of saidrotor magnet; directing an input light beam along an input axis of anoptical assembly which includes said first beam-directing opticalelement, said optical assembly directing said input light beam toprovide an output beam; magnetically biasing said rotor to magneticallyestablish an equilibrium rotational orientation of said rotor andmagnetically impose upon said rotor, when said rotor is rotated fromsaid equilibrium orientation, a torque urging said rotor toward saidequilibrium orientation, said equilibrium orientation of said rotordetermining a neutral axis of said output beam; disposing a coil in alocation with respect to said rotor in which energizing said coilimparts torque to said rotor magnet about said rotation axis; andenergizing said coil with a periodic current, thereby imparting aperiodic motion to said output beam with respect to said neutral axis soas to scan said output beam.
 41. A method according to claim 40, whereinsaid biasing step includes adjusting the position of a ferromagneticelement so as to adjust said equilibrium orientation of said neutralaxis.
 42. A method according to claim 40, wherein said biasing stepincludes establishing a rotational resonant frequency of said rotor. 43.A method according to claim 42, wherein said energizing step includesenergizing said coil with a periodic current at a frequency which isabout said rotational resonant frequency of said rotor.
 44. A methodaccording to claim 42, wherein said energizing step includes energizingsaid coil with a periodic current at a frequency which is substantiallyless than said rotational resonant frequency of said rotor.
 45. A methodaccording to claim 44, wherein said energizing step includes energizingsaid coil with a periodic current having a generally triangularwaveform.
 46. A method according to claim 42, wherein said energizingstep includes energizing said coil with a periodic current producingbetween about 10 and about 200 scans per second of said output beam. 47.A method according to claim 42, wherein said energizing step includesenergizing said coil with a periodic current producing on the order ofabout 40 scans per second of said output beam.
 48. A method according toclaim 40, wherein said coil has a coil axis, and said biasing stepestablishes said equilibrium orientation of said rotor so that saidneutral axis of said output beam is generally parallel to said coilaxis.
 49. A scan element for receiving an input light beam along aninput axis and directing it as a scanning beam with an angularoscillatory movement of said scanning beam about a neutral axis, saidscan element comprising:a coil for carrying electric current, said coilhaving a shape which is generally tubular with a coil axis, an openingalong said coil axis, and an outer peripheral region; a shaft disposedgenerally perpendicular to said coil axis; a rotor mounted for rotationabout a rotation axis determined by said shaft, said rotation axis beingoriented generally perpendicular to said coil axis, said rotor includinga rotor magnet producing a magnetic field oriented generallyperpendicular to said rotation axis, said rotor magnet imparting torqueto said rotor about said rotation axis in response to current in saidcoil; a ferromagnetic circuit including at least one ferromagneticelement, said ferromagnetic circuit being magnetically interactive withsaid rotor magnet so as to establish an equilibrium rotationalorientation of said rotor in which said magnetic field of said rotormagnet is oriented generally perpendicular to said coil axis and so asto impose upon said rotor, when said rotor is rotated from saidequilibrium orientation, a torque urging said rotor toward saidequilibrium orientation, said ferromagnetic circuit including aferromagnetic element having a generally tubular portion located in saidouter peripheral region and oriented generally coaxially with said coil;and an optical assembly disposed to receive an input light beam along aninput axis and direct it as an output beam, said optical assemblyincluding a first beam-directing optical element which is mounted tosaid rotor so as to establish a neutral axis of said output beam whensaid rotor is in said equilibrium orientation and to rotate therewithabout said rotation axis in response to current in said coil and therebyscan said output beam.
 50. A scan element according to claim 49, whereinsaid tubular portion of said ferromagnetic element is at least aboutcoextensive axially with said coil.
 51. A scan element according toclaim 49, wherein said tubular portion of said ferromagnetic element isat least about coextensive axially with said scan element.
 52. A scanelement according to claim 49, further including a bobbin supportingsaid coil, wherein said shaft is mounted to said bobbin.
 53. A scanelement according to claim 52, wherein said shaft is secured to saidbobbin, and said rotor is rotatably mounted on said shaft.
 54. A scanelement according to claim 53, Wherein said rotor includes a rotor mountby which said rotor is rotatably mounted on said shaft, said rotor mountcomprising a lubricated plastic material.
 55. A scan element accordingto claim 52, wherein said shaft is secured to said rotor, and said shaftis rotatably mounted to said bobbin.
 56. A scan element according toclaim 49, wherein said neutral axis is generally parallel to said coilaxis.
 57. A scan element according to claim 49, wherein said input axisis generally parallel to said coil axis.
 58. A scan element according toclaim 49, wherein said neutral axis is generally parallel to said inputaxis.
 59. A scan element according to claim 49, wherein the optical pathof a light beam received and directed by said optical assembly passesthrough said coil opening.
 60. A scan element according to claim 49,wherein said first beam-directing optical element is a mirror which, insaid equilibrium orientation, is oriented diagonally with respect tosaid coil axis.
 61. A scan element according to claim 49, wherein saidoptical assembly includes a second beam-directing optical element, saidfirst and said second beam-directing optical elements each including areflecting surface, said reflecting surfaces of said first and saidsecond beam-directing optical elements being oriented generally parallelto each other.